Florida produces approximately 5 million tons of orange peel waste each year. Most of this peel waste is dried, pelletized, and sold as beef or milk cattle feed filler commonly referred to as citrus pulp pellets.
High gasoline prices, overdependence on foreign oil, and a continuing demand for renewable energy sources have led to increased research interest in the general field of citrus peel waste conversion, and in particular, to the transformation of peel waste to ethanol. Current processes generally involve hydrolyzing citrus peel comprising a complex mixture of polysaccharides to provide fermentable sugars, fermenting the sugars to produce ethanol, and isolating the ethanol and other by-products.
Unfortunately, some compounds found in citrus peel or produced during the steps converting citrus peel into fermentable sugars act as fermentation inhibitors in the conversion of these sugars to ethanol. Among these compounds, limonene, a terpene-based component in citrus peel, is known to impede fermentation processes (See Grohmann, et al., Production of Ethanol from Enzymatically Hydrolyzed Orange Peel by the Yeast Saccharomyces Cerevisiae, Applied Biochemistry and Biotechnology, Vol. 45 (1994)). Limonene is generally understood to provide a natural defense for citrus against bacteria, viruses, molds, and other organisms and to inhibit fermentation by typical processes that would yield ethanol. It has been estimated that, for efficient fermentation, limonene in the citrus peel waste should be below 3000 parts per million and perhaps even below 1500 ppm.
Stewart et al. (US Patent Application No. 2006/0177916) describes a process of producing ethanol from citrus waste where limonene is removed prior to fermentation. The disclosed process includes limonene removal via evaporation and steam stripping from citrus peel, hydrolysis of the limonene-stripped citrus peel waste, and fermentation of the resulting hydrolysis mixture to produce ethanol (or simultaneous hydrolysis and fermentation). The steam-stripped limonene may be recovered by condensation. Stewart does not address the possibility of chemically modifying limonene to reduce or eliminate its fermentation-inhibiting properties.
Cantrall et al. (U.S. Pat. No. 5,186,722) describes that limonene can be used as feedstock for hydrocarbon-based fuels. A variety of products including aromatics (e.g., 1-methyl-4-(1-methylethyl)benzene) and saturated hydrocarbons (e.g., 1-methyl-4-(1-methylethyl)cyclohexane (i.e., menthane)) were produced as mixtures from purified limonene using a hydrogenation catalyst and a variety of other reaction variables (e.g., heat, pressure, and hydrogen). Certain mixtures containing considerable aromatic content were tested and shown to be useful gasoline additives. Cantrall did not specifically test menthane as a gasoline additive. Neither did Cantrall suggest forming menthane in an aqueous mixture of citrus waste, nor the subsequent fermentation of a citrus peel hydrolysate containing certain terpenes that did not substantially inhibit the fermentation. Cantrall also lacks a suggestion that a fermentation beer containing ethanol and certain of these terpenes that do not substantially inhibit the fermentation could be used to provide fuels as an alternative to gasoline.
Inasmuch as there is a continuing demand for alternative and/or renewable energy resources, a need to reduce dependence on foreign oil supplies, and a need to reduce or stabilize gasoline prices for example, there is still an unfulfilled need for a specific and effective solution to address one or more of these issues. In view of the above, it is highly desirable to find new methods of eliminating limonene or other fermentation-inhibiting compounds from citrus waste in order to enhance the production of ethanol therefrom. The present invention is directed to these, as well as other important ends.